1. Field of the Invention
This invention relates to the production of synthetic fuels from coal, heavy oil and similar materials and is particularly directed to a method for the elimination of tar-forming constituents from raw synthesis gas prior to upgrading of the gas.
2. Description of the Prior Art
Conventional gasification processes for the production of synthetic fuels from coal, coke, heavy oil and similar materials generally require reaction of the carbonaceous feed material with steam to produce a synthesis gas containing carbon monoxide and hydrogen as the principal constituents, adjustment of the carbon monoxide-to-hydrogen ratio by passing at least part of the gas stream through a water gas shift reaction unit, and subsequent upgrading of the gas by passing it through a catalytic methanation unit. The raw synthesis gas produced in the initial step of the process generally contains some methane and small amounts of ethane, and some ethylene and other hydrocarbons which tend to undergo polymerization reactions and form tars and resins. Other undesirable materials present in the gas will usually include carbon dioxide, hydrogen sulfide, organic sulfur compounds, phenols, ammonia, hydrocyanic acid, nitric oxide and the like. These constituents tend to reduce the activity of the catalysts used downstream in the process and also lead to the formation of deposits in the equipment employed.
The tar-forming constituents are generally removed from the gas stream before it is introduced into the shift conversion unit. This is usually done by scrubbing the raw gas with water and/or oil to cool it and condense out the tar-forming materials. Tar and naphtha and an aqueous liquor containing phenols and other water-soluble compounds are generally recovered for further processing. The gas stream is then passed through separting devices for the removal of entrained liquids and vapors, reheated, and then introduced into the shift conversion unit for adjustment of the carbon monoxide-to-hydrogen ratio to the desired level. Following this, the gas can be further purified to remove hydrogen sulfide, carbon dioxide and any other contaminants which may adversely affect the methanation catalyst. The methanation step results in the conversion of carbon monoxide and hydrogen to methane and the production of a product gas of sufficiently high B.t.u. content to permit its use as a high grade fuel.
Although processes of the type described above are reasonably effective, such processes have certain disadvantages. Quenching of the raw synthesis gas to cool it and permit the condensation of tar-forming constituents makes it difficult to recover heat from the gas stream and results in relatively low thermal efficiencies. The effluent from this quenching operation usually includes an oily waste water which must be treated for the removal of contaminants before the water can be discharged or reused. The facilities for this represent a significant portion of the total investment and operating costs for such a process. Efforts to avoid these and related problems have in the past been largely unsuccessful.